The efficacy of examination of various cytologic material in patients with mesothelioma was analysed. A total of 48 studies were carried out in 24 patients. Examination of transthoracic aspiration material obtained from the tumor and pleural cavity exudate yielded the best results. When mesothelioma of the pleura is suspected, care should be taken to collect material from different areas of the tumor, bearing in mind the significance of discovering different components of the tumor in it. On grounds of study of various cytologic material, a malignant tumor or adenocarcinoma was diagnosed in 43% and mesothelioma in 48% of patients.